[grantc]

It can be cost-driven, but it's just as likely to be complexity-gated (w/ cost frequently being implementation driven and a proxy for complexity). The cost comes from actually unwinding a gnarly legacy system (that may be less than well understood) and making the successor system work. Also, Gall's law has entered the chat:

A complex system that works is invariably found to have evolved from a simple system that worked. A complex system designed from scratch never works and cannot be patched up to make it work. You have to start over with a working simple system.

[cpeterso]

For folks interested in legacy software systems (and managing the teams managing these systems), I highly recommend Marianne Bellotti's "Kill It with Fire: Manage Aging Computer Systems (and Future Proof Modern Ones)". The author worked at the United States Digital Service.

https://nostarch.com/kill-it-fire

[kjellsbells]

You just described the last 30 years of the US phone network.